FIG. 1 is a circuit diagram of a related DC/DC converter (for example, Japanese Unexamined Patent Application Publication No. 2003-319650). The DC/DC converter illustrated in FIG. 1 is configured as a half-bridge-type current resonant DC/DC converter. Both ends of a DC power source Vdc1 are connected to a series circuit including a switching element Q1 of a MOSFET and a switching element Q2 of a MOSFET.
Between a drain and source of the switching element Q2, a diode D2 and a capacitor C2 are connected in parallel and a series circuit including a resonant reactor Lr, a primary winding 3a of a transformer T, and a resonant capacitor CR is connected. The resonant reactor LR is a leakage inductance between the primary and secondary sides of the transformer T and an exciting inductance is equivalently connected as a reactor LP to the primary winding 3a. Between a drain and source of the switching element Q1, a diode D1 and a capacitor C1 are connected in parallel.
A first end of a secondary winding 3b of the transformer T is connected to an anode of a diode D3. A second end of the secondary winding 3b of the transformer T and a first end of a secondary winding 3c of the transformer T are connected to a first end of a smoothing capacitor CL. A second end of the secondary winding 3c of the transformer T is connected to an anode of a diode D4. A cathode of the diode D3 and a cathode of the diode D4 are connected to a second end of the capacitor CL. Both the ends of the capacitor CL are connected to a load RL.
A control circuit 10a alternately turns on/off the switching elements Q1 and Q2 according to an output voltage Vo from the capacitor CL, thereby conducting PFM control (frequency control) configured to make the output voltage of the capacitor CL constant.
Operation of the related DC/DC converter having such a configuration will be explained in detail with reference to a timing chart illustrated in FIG. 2.
In FIG. 2, Q1v is a drain-source voltage of the switching element Q1, Q1i is a drain current of the switching element Q1, Q2i is a drain current of the switching element Q2, and Io is a rectified output current of the diodes D3 and D4.
From time t10 to t11, the switching element Q1 turns on and the current Q1i passes clockwise through a path extending along Vdc1, Q1, LR, 3a, CR, and Vdc1. Also, the current Io passes clockwise through a path extending along 3b, D3, CL, and 3b. 
Regarding from time t11 to t12, the switching element Q1 turns off and the switching element Q2 turns on. Then, the current Q2i passes counterclockwise through a path extending along CR, 3a, LR, Q2, and CR and the current Io passes counterclockwise through a path extending along 3c, D4, CL, and 3c. 
According to this half-bridge-type current resonant DC/DC converter, a withstand voltage of the switching elements Q1 and Q2 is the power source voltage Vdc1 and a withstand voltage of the diodes D3 and D4 is twice as large as the output voltage Vo. Therefore, low-voltage elements are usable. A zero-voltage switching operation takes place and a current passing through the primary winding 3a is substantially a sinusoidal wave, to realize a highly efficient compact DC/DC converter.
However, as illustrated in FIG. 2, the output current To involves large ripples to cause a problem that the capacitor CL must be enlarged if a large capacity is needed.
To solve the problem, a DC/DC converter illustrated in FIG. 3 may be considered. The DC/DC converter illustrated in FIG. 3 is constituted by connecting three half-bridge-type current resonant DC/DC converters 1a to 1c in parallel. Each of the half-bridge-type current resonant DC/DC converters 1a to 1c is configured nearly the same as the half-bridge-type current resonant DC/DC converter illustrated in FIG. 1 and commonly uses a DC power source Vdc1, a smoothing capacitor CL, and a load RL.
In the DC/DC converter illustrated in FIG. 3, a control circuit 10b individually operates the three half-bridge-type current resonant DC/DC converters 1a to 1c at a phase difference of 120 degrees, to realize large capacity and reduce ripples.